An electrical conductor that carries signals and electrical power through an enclosure or environmental barrier is called a feedthrough. For some applications such as aerospace and defense, the feedthrough must be protected against extreme conditions and be engineered for very high reliability. They require robust seals that maintain electrical integrity and long life environmental sealing. This is the type of application where glass-to-metal-seal technology is best applied. Feedthroughs require
hermetic or airtight seals that can withstand severe conditions such as high temperature, high pressure, vibration, moisture, and difficult chemical environments.
Glass-to-metal-seal (GTMS) feedthroughs have high reliability and high-performance capabilities. Glass-to-metal sealing is a technique to hermetically insulate electrical conductors that pass from one side of a barrier to the other. The glass melt to both package and pin, so it can act as an airtight barrier while at the same time providing the insulation between the housing and the pins. Glass is an outstanding electrical insulator. It also has low thermal conductivity. This combination enables efficient electrical signal and power transmission in severe situations like high pressure and exposure to highly corrosive chemicals.
There are hermetically sealed feedthroughs that are used to conduct power or signals through the metal walls of vessels of liquified gas tanks or through the walls of power plants. The seals for these applications must operate reliably for the long term.
An airtight hermetic seal between glass insulator and a metal conductor or package is called a glass-to-metal seal (GTMS). These seals are commonly used when reliability and long life are important, but the hash use environment makes conventional electronic packaging inadequate.
Classification
Glass-to-metal-seal feedthroughs can be broadly classified as power or signal feedthroughs.
A power feedthrough is mainly to transmit energy, often at high voltage or current. Its conductor is generally large to support 5 to 150 Amps applications. The conductor diameter is in the range of 0.060 inch diameter and up to 0.325 inch Diameter (AWG#0). This kind of feedthrough consists of metal-glass joints wherein the glass serves as an electrical insulation between the connection flange and the conductor. The seal provides isolates the environments on either side of the feedthrough barrier.
Instrumentation feedthroughs, on the other hand, are for electrical signals including thermocouples that are low-current (milliamp), low- or high-voltage signals from low- or high-impedance sources. These conductors are in the range of 0.009 inch, 0.018 inch, 0.020 inch 0.025 inch 0.030 inch diameter to 0.040 inch diameter.
The
RF feedthrough is a special type and is used for high frequency electrical signals. The design is a single pin with a metal shroud calculated on the basis of the glass dielectric constant to achieve a 50 or 75 ohm impedance at frequencies approaching 100 GHz. This kind of feedthrough uses GTMS technology and is often used in commercial and military applications with environments that need reliable and rugged hermetic sealing.
GTMS Process
Glass-to-metal sealing has been in existence for a long time, with the vacuum tubes being one of the earlier applications. The technology is applied to a wide range of uses from glass diodes to hermetic electrical feedthroughs used in vacuum packages. The result is a seal that can withstand high operational pressure or extreme temperature.
Creating a robust seal requires a strong chemical bond between the glass and the metal as well as a matching of the
coefficient of thermal expansion (CTE) between the different materials. CTE is a measure of how the component’s size changes as its temperature varies.
GTMS feedthroughs are formed at high temperatures in excess of 900°C. Management of any thermal mismatch that could happen during the sealing process is important to avoid any residual tension or stress on the joint that could lead to voiding or separation between the glass and the metal.
Ways to Fuse Glass to Metal
Matched Glass-To-Metal-Sealing
Since most applications required devices to operate in a wide range of temperatures, it is vital for the components to expand and contract at very similar rates. This would ensure that no stresses or misalignments occur during production or operation. Feedthroughs must work over the full operating temperature ranges, and generally must work flawlessly over many thousands of thermal cycles.
A common material system used in the industry is Kovar per ASTM F-15 (Fe-Ni-Co alloy) with a Borosilicate glass. The coefficient of thermal expansion of Kovar is a very good match to glass over a wide range of temperatures.
Compression Glass-To-Metal Sealing
If the feedthrough needed focuses on pressure capability, corrosion resistance or conductor strength then a compression type of GTMS may be a better option. Glass has the interesting property of getting stronger when it is compressed. The materials are specifically engineered to result in positive pressure between the glass to metal joints which can enhance overall system strength in difficult mechanical environments.
Applications of GTMS Feedthrough
Industrial Use
Engineers in some advanced industrial applications utilize glass-to-metal seal feedthroughs in the most critical applications. GTMS seals protect sensors from highly corrosive chemicals and excessive high temperature. These feedthroughs have the hermeticity to prevent electronic failure due to extreme environmental conditions. Example applications include humidity, pressure, and flow sensors.
Automotive Industry
Various sensors use GTMS feedthroughs in the automotive industry. Acceleration, gyro, flow and pressure sensors rely on the high-pressure and resistance capability (i.e. vibrations, temperature fluctuations).
Medical Devices
The glass-to-metal seal feedthrough has proven to be effective for
implantable medical devices. The feedthroughs are corrosion resistant and can be biocompatible. Biocompatibility means that the material is compatible with a living tissue and does not produce any toxic or immunological results when exposed to body tissues and fluids.
The use of glass-to-metal seals feedthroughs in medical devices prevent the diffusion of gases which results in degradation of the electronic component or malfunction or corrosion of other parts. In autoclaves, the feedthroughs can withstand shock and high temperature as well as isolate humidity from the autoclave environment.
GTMS feedthroughs are also commonly utilized in endoscopes and surgical instruments. Endoscopes are used to examine the interior of a hollow organ or cavity of the body.
Aerospace and Defense Applications
The aerospace and defense sectors find use for glass-to-metal seal feedthroughs in a wide range of components where system reliability is important: sealed relays, vacuum feedthroughs, pin feed-through terminals, thermal sensors, beam-line feedthroughs and injector tubes, LED's, thermocouple feedthroughs, AC/DC power distribution assemblies, packages for electronic components and time delay devices.
AMETEK Electronic Components and Packaging (ECP) division produces a complete line of hermetically sealed feedthroughs for use in power and signal applications. Products range from µV to kV, and DC to RF applications.
Contact us to discuss your application.
Glossary
CTE – Co-Efficient of Thermal Expansion
a material's fractional change in length for a given unit change in temperature.
Feedthrough
A feed-through is a conductor used to carry a signal through an enclosure or across a mechanical barrier. Like any conductor, it has a small amount of capacitance. A common example of a feed-through connection is an automobile spark plug where the body of the plug must resist the pressure and temperature produced in the engine, while providing a reliable electrical connection to the spark gap in the combustion chamber.
There are electrical hermetically sealed feedthroughs for instrumentation, high amperage and voltage, coaxial, thermocouple and fiber optics. Rotary or mechanical feedthroughs also exist.
Flange
A projecting rim or collar on a package base or lid.
Hermetics
completely sealed, especially against the escape or entry of air.
Hermetic Seals
prevent the invasion of oxygen, moisture, humidity, and any outside contaminant to enter a sealed environment.
Kovar
Iron Nickel Alloy with the addition of Cobalt as a hardener.
Matched Seal
A GTMS in which all of the components have approximately the same thermal expansion coefficient and therefore expand and contract at approximately the same rate of speed. Hermeticity is achieved by a molecular bond between the glass and metals. Also known as a chemical or molecular bond seal.
Thermal Conductivity
The rate at which a material conducts heat.